This invention relates to a.c. solid state relays, and more specifically relates to a novel solid state relay circuit and housing therefor.
Solid state a.c. relays are well known. Such relays, with optical isolation between input and output, are also well known. In presently existing devices, many discrete components are commonly required to complete the a.c. circuit. Thus, it may take thirty or more discrete thyristors, transistors, resistors and capacitors to manufacture a single device. Attempts have been made to integrate the various parts of the entire solid state relay, but these have met only limited success due to the mix of high voltage and high power components.
Solid state relays made in the past have also employed zero voltage crossing circuits to ensure turn on of the thyristor only when the a.c. voltage is within some small "window". These circuits have also been relatively complex and difficult to integrate into the main power chip. Thus, zero cross firing circuits have required the use of a discrete resistor connected across the power terminals. These resistors have not been easily integrated into a single chip because of the difficulty of forming this resistor on the chip surface.
It has also been difficult to provide so-called "snubberless" operation for the relay under any inductive or resistive load. Thus, while solid state relays may operate well under resistive or slightly inductive loads, they may tend to "half wave" or "chatter", which is a condition wherein a relay turns on only for one-half of a cycle, under a highly inductive load. This has occurred in the past because the relays are commonly provided with conditioning circuits for suppressing fast turn on of the circuit under some fast transient or high dV/dt condition. When the device is operated under a very highly inductive load, however, voltage transients are commonly generated repetitively during device turn on. When the signal conditioning circuit misinterprets this as a transient signal, it shuts off the power output during a particular half phase of the operation. The circuit will then appear to turn to normal during the next half wave and the relay will turn on. This condition repeats so that the relay turns on only during one or another of the half waves of the full cycle. To avoid this condition, relays of the past have been formed with reduced firing sensitivity and this has required reduction of sensitivity to optical firing.
Since prior art relays have been relatively complex, they have required substantial volume for their housings. Moreover, solid state relays of the past have been limited to a maximum temperature rise of about 110.degree. C., thus limiting their current-handling capability. Finally, solid state relays of the past have been relatively expensive in view of the need for large numbers of discrete components and large housings.